The present invention relates to image forming apparatus employing an electro-photographic method, such as copiers, printers, facsimile devices, etc., in which a charging means, an image writing means and a developing means are disposed around the circumferential surface of an image bearing member to form a toner image, which is transferred and fixed on a transfer sheet, and specifically relates to a duplex image forming apparatus which can form images on both sides of a transfer sheet by employing an intermediate transfer belt.
In a conventional duplex image forming apparatus, a toner image of one side, formed on the image bearing member, is transferred and fixed on the transfer sheet to be temporarily stored in the reversible feeding device after the fixing operation. Then, the transfer sheet is fed from the reversible feeding device, synchronized with another toner image formed again on the image-bearing member, to transfer and fix the other toner image onto the other side of the transfer sheet.
In the conventional duplex image forming apparatus, since the transfer sheet should be conveyed to the reversible feeding device and should be pass through the fixing device twice, as mentioned above, the transfer sheet is compelled to pass through a very complicated conveyance path, which deteriorates a reliability of the conveying operation for the transfer sheet and causes jams and wrinkles of the transfer sheet.
To overcome the abovementioned problems, the duplex image forming apparatus, in which toner images, formed on the both sides of the transfer sheet by employing both the image bearing member and the intermediate transfer member, are fixed onto the transfer sheet at a time, are set forth in Tokkaishou 49-37538, Tokkaishou 54-28740, Tokkaihei 1-44457 and Tokkaihei 4-21576.
Further, the present inventors have disclosed the method and apparatus for forming duplex color images set forth in Tokkaihei 9-258492 and Tokkaihei 9-258516, in which a plurality of sets, each of which is comprised of a charging means, an image writing means and a developing means, are arranged around the circumferential surface of the photoreceptor drum, and the superimposed color toner image formed on the photoreceptor drum is collectively transferred onto the intermediate transfer member, and then, the transfer sheet is conveyed on the intermediate transfer member, synchronizing with the superimposed color toner image formed again on the photoreceptor drum and the toner image already transferred on the intermediate transfer member, and during the conveying operation of the transfer sheet, the toner image formed again on the photoreceptor drum is transferred onto one side of the transfer sheet as the obverse side image while the toner image already transferred on the intermediate transfer member is transferred onto the other side of the transfer sheet as the reverse side image, and then, the transfer sheet is separated from the intermediate transfer member to fix the toner images onto the transfer sheet at a time, and thus, the duplex color images can be formed.
In the duplex image forming apparatus which forms images on both sides of the transfer sheet by employing an intermediate transfer belt, serving as a belt-shaped intermediate transfer member as mentioned above, there are provided a transfer sheet charging means for charging the transfer sheet, such as a recording sheet, etc., to convey the transfer sheet along the intermediate transfer belt while the transfer sheet is electro-statically adhering onto it, a first transfer means for transferring a toner image, formed on the image bearing member, onto the intermediate transfer belt or the obverse side of the transfer sheet, and a second transfer means for re-transferring the toner image, already transferred on the intermediate transfer belt, onto the reverse side of the transfer sheet.
When the transfer sheet is conveyed on the intermediate transfer belt during the duplex image forming operation, a bias voltage is applied to the transfer sheet charging means, the first transfer means and the second transfer means, in order to transfer good toner images onto the both sides of the transfer sheet. Incidentally, in the system for forming the abovementioned duplex images, the transfer sheet charging means, the first transfer means and the second transfer means are disposed at positions as described in the following.
Initially, the first transfer means is disposed at a position backside the intermediate transfer belt, where the photoreceptor drum opposes to the intermediate transfer belt, to supply electronic charge onto the backside surface of the intermediate transfer belt (for instance, the lower side surface of the transfer sheet shown in FIG. 1). In this configuration, a toner image formed on the photoreceptor drum is transferred onto one side of the transfer sheet (for instance, the upper side surface of the transfer sheet shown in FIG. 1), which is conveyed by the intermediate transfer belt while electro-statically adhering on it.
On the other hand, the second transfer means is disposed at a position, located downstream the position of the first transfer means and at a front-side of the intermediate transfer belt, to directly supply electronic charge onto the opposite side surface of the transfer sheet (for instance, the upper side surface of the transfer sheet shown in FIG. 1), which is conveyed by the intermediate transfer belt while electro-statically adhering on it. In this configuration, a toner image already transferred on the intermediate transfer belt is transferred onto the other side of the transfer sheet (for instance, the lower side surface of the transfer sheet shown in FIG. 1).
Further, the transfer sheet charging means is disposed at a position, located upstream the position of the first transfer means and at a front-side the intermediate transfer belt, to directly supply electronic charge onto the upper side surface of the transfer sheet, shown in FIG. 1, which is fed on the intermediate transfer belt, so that the transfer sheet electro-statically adheres onto the intermediate transfer belt.
Accordingly, in the system for forming the duplex images on the both sides of the transfer sheet at a time, the first transfer means indirectly supplies electronic charge to one side of the transfer sheet with putting the intermediate transfer belt between them, while the second transfer means or the transfer sheet charging means directly supplies electronic charge to the other side (opposite side) of the transfer sheet.
The present inventors have found a problem that the electronic charge, applied to one side of the transfer sheet, and the other electronic charge, applied to the other side of the transfer sheet, interfere each other in the system mentioned above. Namely, the present inventors have found that, since the relative position of the transfer sheet, with respect to the positions of the transfer sheet charging means, the first transfer means and the second transfer means, varies depending on the current position of the transfer sheet as convying with the intermediate transfer belt, presence or absence of the interference between electronic charges supplied to the both sides of the transfer sheet considerably affects the quality of the toner images.
Specifically in the duplex image forming apparatus, incorporating the intermediate transfer belt, it is also found that a delicate variation of the electronic resistance of the transfer sheet or that of the intermediate transfer belt considerably affects the interfering action between electronic charges residing on the both sides of the transfer sheet. The abovementioned interfering action gives far greater influences to the transfer sheet than that in image forming apparatus, which forms a toner image on only one side of the transfer sheet.
Owing to the interfering action in the abovementioned duplex image forming apparatus, there has been a problem that an unevenness of the density like a stripped pattern is generated in the transferred images, when the ambient humidity is high, or the transfer sheet, having a low resistance, is utilized, though good transferred images can be obtained under the normal conditions.
To overcome the abovementioned drawbacks in conventional duplex color image forming apparatus, it is an object of the present invention to provide a duplex color image forming apparatus, in which a transferring operation of toner images is performed at a constant transferring efficiency, even if an ambient humidity is high, or a transfer sheet, having a low resistance characteristic, is utilized, and which makes it possible to form uniform and high-quality color images without generating any unevenness of the density like a stripped pattern.
Accordingly, to overcome the cited shortcomings, the abovementioned object of the present invention can be attained by duplex image forming apparatus described as follow.
(1) A duplex image forming apparatus, comprising: an image bearing member; a toner image forming section to form a toner image on the image bearing member; an intermediate transfer element, shaped like a belt and having a first surface onto which the toner image is transferred from the image bearing member and a second surface located opposite the first surface, to bear the toner image transferred from the image bearing member and to convey a transfer sheet while holding it on the first surface; a transfer sheet charging device, disposed opposite a charging position and facing the first surface of the intermediate transfer element, to apply a bias voltage onto the transfer sheet at the charging position, so that the transfer sheet adheres onto the first surface of the intermediate transfer element; a first transfer device, disposed opposite a first transferring position and facing the second surface of the intermediate transfer element, to transfer the toner image, formed on the image bearing member, onto the transfer sheet or the intermediate transfer element at the first transferring position; a second transfer device, disposed opposite a second transferring position and facing the first surface of the intermediate transfer element, to transfer the toner image, borne on the first surface of the intermediate transfer element, onto the transfer sheet at the second transferring position; and a control section to control a transfer current or a transfer voltage applied to the first transfer device, wherein, under a condition that a length of the transfer sheet in its conveyance direction is longer than a length of a conveyance path ranging from the charging position to the second transferring position, the control section changes the transfer current or the transfer voltage, corresponding to each of three states, including a first state in which a leading edge of the transfer sheet has not arrived at the second transferring position, a second state in which the leading edge of the transfer sheet has passed through the second transferring position while a trailing edge of the transfer sheet has not arrived at the charging position and a third state in which the trailing edge of the transfer sheet has passed through the charging position.
(2) The duplex image forming apparatus of item 1, wherein the control section changes an absolute value of the transfer current or the transfer voltage in the second state into a smaller absolute value than that of the transfer current or the transfer voltage applied to the first transfer device in the first state.
(3) The duplex image forming apparatus of item 1, wherein the control section changes an absolute value of the transfer current or the transfer voltage in the second state into a larger absolute value than that of the transfer current or the transfer voltage applied to the first transfer device in the third state.
(4) The duplex image forming apparatus of item 1, further comprising: a leading edge detecting device to detect a passage of the leading edge of the transfer sheet; and a trailing edge detecting device to detect a passage of the trailing edge of the transfer sheet, wherein the control section changes the transfer current or the transfer voltage based on signals detected by the leading edge detecting device and the trailing edge detecting device.
(5) The duplex image forming apparatus of item 4, wherein the leading edge detecting device and the trailing edge detecting device detect the passage of the leading edge of the transfer sheet and the passage of the trailing edge of the transfer sheet, respectively, by detecting a change of a charge current or a charge voltage of the transfer sheet charging device.
(6) The duplex image forming apparatus of item 1, further comprising: a detecting device to detect a electronic resistance of the transfer sheet, or to detect a humidity in the duplex image forming apparatus, wherein the control section changes the transfer current or the transfer voltage applied to the first transfer device, when the electronic resistance, detected by the detecting device, is lower than a predetermined resistance value, or when the humidity, detected by the detecting device, is higher than a predetermined humidity value, while the control section does not change the transfer current or the transfer voltage applied to the first transfer device, when the electronic resistance, detected by the detecting device, is higher than the predetermined resistance value, or when the humidity, detected by the detecting device, is lower than the predetermined humidity value.
(7) The duplex image forming apparatus of item 6, wherein the control section further changes the transfer current or the transfer voltage applied to the first transfer device in each of the three states, including the first state, the second state and the third state, corresponding to a detecting result of the detecting device.
(8) The duplex image forming apparatus of item 1, further comprising: a second image bearing member; and a second toner image forming section to form a second toner image on the second image bearing member, wherein the second toner image, formed on the second image bearing member, is transferred onto the first surface of the intermediate transfer element.
(9) A duplex image forming apparatus, comprising: an image bearing member; a toner image forming section to form a toner image on the image bearing member; an intermediate transfer element, shaped like a belt and having a first surface onto which the toner image is transferred from the image bearing member and a second surface located opposite the first surface, to bear the toner image transferred from the image bearing member and to convey a transfer sheet while holding it on the first surface; a transfer sheet charging device, disposed opposite a charging position and facing the first surface of the intermediate transfer element, to apply a bias voltage onto the transfer sheet at the charging position, so that the transfer sheet adheres onto the first surface of the intermediate transfer element; a first transfer device, disposed opposite a first transferring position and facing the second surface of the intermediate transfer element, to transfer the toner image, formed on the image bearing member, onto the transfer sheet or the intermediate transfer element at the first transferring position; a second transfer device, disposed opposite a second transferring position and facing the first surface of the intermediate transfer element, to transfer the toner image, borne on the first surface of the intermediate transfer element, onto the transfer sheet at the second transferring position; and a control section to control a transfer current or a transfer voltage applied to the first transfer device, wherein, under a condition that a length of the transfer sheet in its conveyance direction is longer than a length of a conveyance path ranging from the charging position or the second transferring position to the first transferring position, the control section changes the transfer current or the transfer voltage, corresponding to each of two states, including a first state in which the transfer sheet resides at both the first transferring position and at least one of the charging position or the second transferring position, and a second state in which the transfer sheet resides only at the first transferring position.
(10) The duplex image forming apparatus of item 9, wherein the control section changes an absolute value of the transfer current or the transfer voltage in the first state into a smaller absolute value than that of the transfer current or the transfer voltage applied to the first transfer device in the second state.
(11) The duplex image forming apparatus of item 9, further comprising: a leading edge detecting device to detect a passage of the leading edge of the transfer sheet, wherein the control section changes the transfer current or the transfer voltage, applied to the first transfer device, based on signals detected by the leading edge detecting device.
(12) The duplex image forming apparatus of item 11, wherein the leading edge detecting device detects the passage of the leading edge of the transfer sheet by detecting a change of a charge current or a charge voltage of the transfer sheet charging device.
(13) The duplex image forming apparatus of item 9, further comprising: a second image bearing member; and a second toner image forming section to form a second toner image on the second image bearing member, wherein the second toner image, formed on the second image bearing member, is transferred onto the first surface of the intermediate transfer element.